Method of recovering useful compounds from anthracene oil



March 13, 1962 GlLBERT 3,025,304

METHOD OF RECOVERING USEFUL COMPOUNDS FROM ANTHRACENE OIL Filed March '7, 1960 2 CONDENSER 8 /2 BED FLASH f .ST/LL 9 co/vomsm 7 a .L, /0 /4 f Xylene BED if; H CRYSTALL/ZER l5 6 Steam [74 &

//v l/E/V r05 GER/1L D GILBERT Attorney 3,025,304 METHQD F RECGVERING USEFUL COM- POUNDS FROM ANTHRACENE OIL Gerald Gilbert, Pittsburgh, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Mar. 7, 1960, Ser. No. 13,269 3 Claims. ((31. 260318) This invention relates to a method of recovering from anthracene oil certain useful compounds present therein in low concentration. Carbazole is typical of such compounds.

No satisfactory process has heretofore been available, so far as I am aware, for recovering certain components of anthracene oil which are present in low concentrations. Carbazole is an example. While it is used commercially for a variety of purposes, it is present in anthracene oil in concentrations of only about The unstable character of many of the components of anthracene oil and the low concentration of carbazole largely explain the difficulties encountered in separating the latter in good yield and high purity. Even more important, the complexity of the mixture and the numerous interactions that can occur between components cause ditliculty in predicting the effectiveness of any given process. Many processes for the isolation of carbazole from coal tar or a fraction thereof have been reported. In most of these, the yield or purity of the product is low or the process is too complex for efficient operation. Examples of the methods used are (1) multiple distillations or crystallizations; (2) sublimation; (3) acid or solvent extraction; and (4) reaction of the anthracene-oil fraction with an alkali. It is therefore the object of this invention to provide a method for recovering carbazole which is inexpensive, efficient and easy to practice.

In general terms, my method comprises, as a first stage, vaporizing anthracene oil at reduced pressure and bringing the vapor into contact with a base deposited on a porous medium, thereby causing the formation and deposition of a salt (carbazolate) thereon. In a second stage, I pass steam through the porous medium, thereby hydrolyzing the salt and carrying off the carbazole vapor with the steam. Condensation of the vapor and steam permit ready separation of the resulting water and carbazole.

The absorbent medium is prepared by impregnating a carrier such as asbestos with a base such as potassium hydroxide. This material is packed into a tube and the carbazole containing material passed through in the gas phase. At the temperature of operation, the c-arbazole remains in the vapor phase, but the potassium carbazolate formed is a solid and remains on the absorbent. The Water formed in this reaction is vaporized rapidly under the conditions of operation and leaves the tube in the gas stream. Since most other materials associated with carbazole in coal tar and other mixtures are not acidic, they do not form salts but remain in the gas stream to pass out of the end of the tube with the water formed in the above reaction. Thus, in the first cycle of this process, potassium carbazolate is formed in high con centration in the reactor tube, but other associated ma terials remain in the gas stream and may be condensed and separated at the end of the tube. In the second step of the process, steam is passed through the bed containing the potassium carbazclate. The carbazole thus formed will vaporize under the conditions of operation and be carried from the end of the tube to be separated from the water and up raded to the desired commercial purity.

The process of the present invention can be further illustrated by reference to the drawing which represents diagrammatically a commercial unit for the production of carbazole. In operating this unit, the carbazole-conire States atent taining starting material, such as anthracene oil, is introduced in the liquid phase to a flash still 1 through line 7. In still 1, the oil is vaporized and the vapor is then passed through line 8 into a reaction bed 2. Bed 2 is composed of a loosely packed absorbent medium, prepared by mixiiug asbestos with a concentrated solution of potassium hydroxide and allowing the water to evaporate. As the vapors pass through bed 2, the carbazole is selectively absorbed and the other components of anthracene oil pass out by line 12, are condensed and carried out of the system through line 18 for use in other processes or as fuel.

In the second step, the vapor is diverted through line 10 to bed 3, which is similar to bed 2. In bed 3, the carbazole is again selectively absorbed and the remaining components are discharged through line 14. Concurrently, steam is passed through bed 2, entering through line 9 to regenerate the carbazole by hydrolysis. The vapor thereof arising from bed 2 flows through line 13 as a mixture with steam. This mixture is condensed in condenser 5, the water is separated and the carbazole is discharged through line 16 to crystallizer 6 in which it is upgraded to commercial purity and discharged through line 17. The upgrading process generally consists of one crystallization from xylene. After bed 2 has been regenerated with steam, the carbazole vapor from the flash still 1 is again passed through it, and bed 3 is then regenerated by passing steam through line 11. The carbazole-water mixture emerges from bed 3 through line 15 and is condensed in condenser 5. The water is then decanted, and the carbazole is discharged through line 16, crystallized in crystallizer 6, and discharged through line 17. This system permits the continuous treatment of a coal-tar stream and the continuous production of carbazole from a starting material of low carbazole concentration.

The process of the present invention can be further illustrated by the following examples.

Example I A glass tube 2 feet long and /2 inch in diameter was attached at its upper end to a standard distilling flask and at its lower end to a distillation receiver fitted for reduced pressure. The apparatus was similar to the normal laboratory equipment for reduced-pressure distillation, with the glass tube replacing a condenser. The glass tube was placed in a column wound with nichrome wire for efficient heating. Thermocouples were introduced at proper intervals to record the temperature of the column during operation. The distilling flask and distilling head were wrapped with heating tape for greater efficiency in vaporizing the sample into the tube. In order to raise the vapor stream to the desired reaction temperature, the first part of the tube was wrapped for greater heating capacity than the latter part.

The column packing was prepared by mixing 100 grams of potassium hydroxide, grams of asbestos and 250 grams of water, and evaporating the water under reduced pressure. This mixture was packed into the reaction tube, except for the topmost several inches of the tube, which were packed with untreated asbestos to act as an introductory section for preheating the vapor to the desired reaction temperature before contacting it with the potassium hydroxide. A 31.6 gram sample of anthracene oil boiling between 315 and 355 C. and containing 15% carbazole, was placed in the distillation flask and the system exacuated to 50 millimeters pressure.

The distilling flask was then heated to vaporize the carbazole charge and pass it into the absorbent, which was kept at 300 C. 82% of the material charged distilled through the absorbent bed apparently unchanged. The remainder of the material was trapped in the bed. In the second step, the distillation flask and head were removed and steam was passed through the bed at atmospheric pressure. The steam-carbazole mixture issuing from the bed was condensed and the water separated by decantation or filtration. The carbazole obtained was 92% pure. Of the carbazole in the starting material, 81% was recovered as product.

Example I] The same crude carbazole used in Example I was again charged to the distillation flask. The charge amounted to 36.8 grams. The system was reduced to 50 millimeters pressure and the charge was distilled through the packed bed prepared as in Example I. In this run, the bed was kept at 280 C. during the passage of the impure carbazole. Of the charged material, 75.7% distilled in the first phase of the process. The regeneration process was repeated as in Example I, and it yielded a product containing 88.2% carbazole and representing 61% of the carbazole in the impure fraction. The initial distillate contained only 0.3% carbazole or approximately 1.4% of the original carbazole present.

Example 111 The process of Example I was repeated with the bed at 320 C. The amount of distillate obtained in the first step was 78.2% of the total charge. The product contained 87% carbazole and represented 74% of the carbazole in the impure fraction.

Operation at pressures lower than 50 millimeters materially reduces the yield of carbazole. It is, of course, necessary to keep the reaction pressure at such a point that the carbazole will boil below the reaction temperature and may be vaporized into the reactor.

Since my process is based on the hydrolytic reversal of the salt formation of very weak acids, it is not limited to carbazole but may be applied to any weak acid that may be regenerated or hydrolyzed readily at elevated temperatures. Thus, other substances that may be separated by this method are pyrrole, indole, diphenylamine, and such acidic hydrocarbons as indene, fiuorene, triphenylmethane, xanthenes, and acetylenes. The stability of these compounds at the temperatures of operation will,

of course, affect the utility of the present process for their isolation. My process, furthermore, is not limited to the use of potassium hydroxide, although this alkali has been found preferable for isolation of carbazole. Any base can be used that forms a readily hydrolyzable salt at elevated temperatures with the substance to be isolated.

It will be evident from the foregoing that the invention affords a simple effective method for recovering carbazole from anthracene oil. It thus makes the product available at a lower cost than heretofore.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as well any change or modification therein which may be made Without depart--.

ing from the spirit and scope of the invention.

I claim:

1. A method of recovering carbazole from anthracene oil which comprises forming a gas-permeable bed of fibrous material, depositing a layer of an alkali on the material of said bed from an aqueous solution thereof, vaporizing anthracene oil under a partial vacuum, passing the resulting vapor through said bed until a deposit of carbazolate builds up thereon, then passing steam through said bed to hydrolyze and vaporize the carbazolate, condensing the resulting vapor and separating the carbazole from the aqueous condensate.

2. A method as defined by claim 1, characterized by said vacuum being about millimeters of mercury.

3. A method as defined by claim 1, characterized by said bed being heated to a temperature of about 300 C.

References Cited in the file of this patent UNITED STATES PATENTS 

